An MD explains the causes, symptoms, and what the diagnosis really means about a person’s health.

Hillary Clinton is being treated for pneumonia, her campaign announced yesterday after video cameras captured her losing her footing and being helped into a van in New York City. The Democratic presidential nominee felt overheated and was suffering from dehydration, Clinton’s doctor also said in a written statement, but by Sunday evening she was “recovering nicely.”

Pneumonia is very common—but that doesn’t mean there aren’t misconceptions about the causes and symptoms, and what getting the illness really means for a person’s overall health. Here, Michael Niederman, MD, American Thoracic Society member and professor of clinical medicine at Weill Cornell Medical College, fills Health in on all things pneumonia, and what factors the American public should focus on as Clinton recovers.

Pneumonia Isn’t Always as Scary as it Sounds

First things first: Pneumonia is an infection in one or both lungs caused by bacteria, a virus, or fungi. It can cause difficulty breathing and can be life threatening, especially in people with weakened immune systems or other chronic health conditions.

But many cases of pneumonia are mild and can be treated during a regular doctor’s visit. “When it arises outside of the hospital, those patients generally do get better quickly,” says Dr. Niederman. (Hospital-acquired pneumonia tends to be more serious.) “They usually don’t end up needing advanced therapy besides rest, hydration, and antibiotics.”

The only way to know for sure if an illness is pneumonia is to take a chest X-ray, but doctors often diagnose it and prescribe antibiotics after examining patients and listening to their lungs with a stethoscope.

Clinton’s diagnosis was made on Friday during a follow-up evaluation of a prolonged cough related to allergies, said Lisa Bardack, MD, the former Secretary of State’s personal physician, in a statement. “She was put on antibiotics, and advised to rest and modify her schedule,” Dr. Bardack’s statement reads. (Clinton has since canceled a trip to California planned for this week.)

The Classic Symptoms Mean Your Immune System is Working

The way pneumonia presents itself has a lot to do with a person’s overall health, says Dr. Niederman. “If you’re in good physical condition, you typically get more classic symptoms like fever, chills, chest pains, or some shortness of breath,” he says.

Elderly people or those who are chronically sick may have more subtle symptoms, like confusion and weakness, without specific respiratory complaints.

“You would think the people who look sickest are the ones who are the worst off, but it really is the opposite,” Dr. Niederman says. “The people who look sickest are having a good immune response to the infection.”

Dehydration is a Common Side Effect

Clinton, 68, attended an outdoor memorial service for 9/11 victims at Ground Zero yesterday, where she stood in a crowd for about 90 minutes. She left the ceremony early—stumbling on her way out—and spent a few hours at her daughter’s nearby apartment before facing the press again.

Dr. Niederman, who has not treated Clinton, says it makes sense that someone with pneumonia could start to feel weak or ill at such an event.

“I’m going to guess that she had a low-grade fever, and dehydration is also common in pneumonia because you generally don’t feel well and don’t feel like eating and drinking,” he said. “If you’re running a fever or sweating, it’s even more likely to get dehydrated, and I think that’s probably what happened to her.”

The fact that Clinton appeared much better later in the day also points to dehydration, Dr. Niederman says.

“Three our four hours later she’s up and walking around, and the only thing I can think of that would make her able to recover that quickly is fluids,” he explains. “If she had really severe pneumonia or something else really serious, I don’t think we’d see her standing up, walking, and talking like normal so soon.”

There is a Pneumonia Vaccine, But it Doesn’t Protect Against All Strains

“One comment that’s come up in the news is that there’s a vaccine for pneumonia, and if she had gotten a vaccine this never would have happened,” says Dr. Niederman. “But the truth is that the vaccine is for only one type of pneumonia—the kind caused by Streptococcus pneumoniae bacteria.”

That’s the most common type of pneumonia, but it still only accounts for about 30% to 40% of all cases. “Even if she had a vaccine—and she very well may have—it’s still possible to get sick,” he says.

In fact, Dr. Niederman adds, getting pneumonia isn’t necessarily a reflection of being unvaccinated—nor is it a reflection of being exhausted or in poor physical health. “Anyone can get pneumonia if they’re exposed to bacteria they’re not immune to,” he says. “The idea that you get it because you’re tired and run down is mostly folklore.”

For Healthy People, Recovery is Usually Fast and Full

Adults over 65 are at increased risk for complications from pneumonia, and serious cases that require hospitalization have been linked to long-term health issues like cardiovascular tissue damage and an increased risk of heart disease. But most healthy patients who respond to antibiotics recover “fully and rapidly,” says Dr. Niederman.

For now, says Dr. Niederman, there’s no reason to suspect this won’t be the case for Clinton. “Well see over the next few days how quickly she recovers,” he says. “In retrospect, it will give us an idea how serious or how mild her pneumonia really was.”